The present invention relates to methods of fabricating structured catalyst monoliths and, more particularly, to methods for processing structured catalyst supports of alumina composition to deposit active catalytic materials on the surfaces of the supports.
Currently a wide variety of pellet-type catalyst structures formed of gamma alumina or other oxides, e.g., pellets, pills, beads, rings, trilobes, stars or the like are used as catalysts or catalyst supports in the petrochemical industry. These structures are typically formed by extrusion from batch mixtures of alumina or other selected oxides, followed by drying and calcining. The objective is to produce shapes which are crush- and attrition-resistant when packed into reactor beds.
Although technologies for making durable, active pellet-type alumina supports or catalysts are well developed, such structures are not optimally configured for most catalytic reactor applications. Pellet beds tend to exhibit relatively high flow resistance in comparison with honeycomb supports, and also develop preferential flow paths which exhaust portions of the catalyst while leaving other portions relatively unused.
Some of these problems have been addressed, particularly for gas-phase reactions, through the adoption of ceramic honeycombs as physical structural supports for chemically active, high-surface-area catalysts or catalyst support coatings. U.S. Pat. No. 4,965,243, for example, describes coated honeycomb support structures provided with high-surface-area washcoatings of gamma-alumina supporting metal catalysts for the treatment of automobile exhaust gases.
However, there are a number of applications for which porous washcoatings are inadequate from the standpoint of durability and/or catalyst loading capacity, and for those applications catalysts or catalyst supports made up mostly or entirely of active, high-surface-area material must instead be used. Such applications include chemical processes wherein the kinetics of the chemical reaction(s) on the catalyst are slow relative to the diffusion and mass transfer steps involved in the overall process.
An example of such an application is the hydro-desulfurization of fossil fuels in the petrochemical industry to make low sulfur gasoline and diesel fuels. Since the reaction kinetics are the slow step in such processes, it is important to provide a relatively large accessible BET catalyst support surface (more catalyst sites in a given volume) in order to allow the most effective use of reactor volume. This in turn requires that the entire volume of the catalyst or catalyst support structure be made of active, high-surface-area material, and that the pore structure of the material be such that that the reactants can diffuse in and products diffuse out of the volume of the catalyst support effectively over relatively long distances. Examples of alumina honeycombs that can be useful in such applications are disclosed in U.S. Pat. Nos. 4,631,267 and 6,365,259.
Alumina honeycombs such as described in these patents can be relatively large in comparison to conventional bead or pellet catalyst supports. For physical durability, therefore, a premium is placed on developing high strength as well as high surface area in the finished alumina honeycomb product. However, owing to that high surface area, which in many cases includes a high degree of mesoporosity, extruded alumina monoliths can adsorb large amounts of water. This creates a susceptibility to cracking when the alumina surface is first highly dehydrated, and then subjected to aqueous vapors or solutions.
This susceptibility to cracking is particularly problematic in cases where it is necessary to deposit additional catalytic materials onto the channel wall surfaces of the honeycombs. Typically, both pelletized and monolithic γ-alumina catalyst supports are catalyzed by impregnation with aqueous solutions of catalyst salts in order to develop the desired concentration of catalysts on the supports. This process normally effects the complete wetting of the alumina surface and the filling of the pores of the alumina honeycomb walls with catalyst solution.
In the case of alumina honeycombs, it has been found that the extrusion, drying, and calcination steps typically involved in the production of high-surface-area alumina honeycombs creates highly dehydrated external and internal surfaces. In this state, water adsorption onto the alumina surface and/or water intrusion into the pore structure of the honeycomb walls are particularly rapid and complete. Even physical absorption of water from the vapor phase into the highly porous material can be significant.
As a consequence of this honeycomb characteristic, during the impregnation of alumina honeycombs with aqueous solutions of catalyst precursor salts, immersing a very well dried, water deficient honeycomb causes rapid adsorption of water into the pore structure of the ceramic. In addition, surface hydroxyl groups form from the wetting and from dissocative adsorption of water onto the alumina during chemisorption.
Such water adsorption does not create catastrophic mechanical loss in the case of alumina pellets, beads or other small catalyst support shapes. This is attributed to the small size and substantial cross-sectional thickness of such shapes which enables them to withstand or accommodate the filling and swelling of the pore structure. Particles and beads are also able to withstand other structural changes as well as localized exothermic effects of water absorption.
In the case of highly dehydrated alumina honeycombs or other thin-walled monoliths, and especially in the case of dehydrated honeycombs formed of transition aluminas such as gamma-alumina, however, cracking can easily occur during exposure to these aqueous solutions. This is due not only to the high mesoporosity and high surface area of these honeycombs, but also because of a relatively inflexible geometric structure consisting largely of a matrix of relatively thin, brittle ceramic walls. Apparently the dimensional changes that can occur as a consequence of such treatments, e.g., impregnating the honeycombs with a catalyst solution or otherwise contacting the honeycombs with water, can initiate structural cracking of the monoliths at a level that can in many cases significantly reduce the strength and/or physical durability of the honeycombs.